It is known to heat steel into the austenitic region followed by quenching sufficiently to avoid the formation of ferrite, pearlite, and bainite. Hypoeutectoid steels are quenched from a temperature in excess of 1600.degree. F., whereas hypereutectoid steels are quenched from a temperature in excess of 1340.degree. F. Various alloys inhibit the formation of decomposition products and allow considerably slower cooling rates and therefore is advantageous in hardening of thick pieces of steel.
Accordingly, it is well known in the art to heat-treat steels by heating the material to a temperature in excess of 1670.degree. F. and thereafter rapidly cooling the metal so as to avoid the formation of decomposition products.
It is often desirable, especially on gears and sprockets, to harden or temper the outer marginal surface area of the teeth and to leave the main body or central portion in an annealed or normalized condition. The heat treated sprocket teeth will exhibit resistance to abrasive wear while the untreated main central body portion of the sprocket will exhibit a crystalline structure comprised of a relatively smaller grain size. Hence, the sprocket's main body portion will be free of stresses and less likely to develop cracks and other flaws during usage.
It is known to heat-treat the entire sprocket by impinging a flame on the surface of the teeth so as to form austenitic steel, and to rapidly quench the entire sprocket by submersion in a liquid, either water or oil. This prior art hardening process is objectionable because the central body portion of the sprocket acts as a heat sink, and accordingly, it too is heated sufficiently to lose its original ductility. Accordingly, it is desirable to be able to take advantage of the heat sink properties of a sprocket so that the central body portion thereof remains ductile while at the same time the sprocket teeth are hardened so that they loose their ductility and become tempered, thereby attaining the maximum advantages from the metallic properties of the sprocket.